-cm 




ractical Directions for 
Marking and Adjusting 

THE CORLISS 

VALVE GEAR 

With One and Two Eccentrics 
By H. H. KELLEY 



W. R. C. SMITH PUBLISHING CO. 

Atlanta 1908 Georgia 



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ADJUSTING THE CORLISS VALVE GEAR. 



Single Eccentric Gear. 

IN preparing the following instructions for setting and adjust- 
ing the standard type of Corliss valve gear, both single 
and double eccentric, it has been assumed that the reader 
is familiar with the construction of the standard form of Corliss 
gear, and with the operation of the gear when it is in proper 
working order. Having that much information to start with, 
the reader will find little difficulty in setting almost any make 
of Corliss engine valve gear, provided he will follow the instruc- 
tions carefully and will carry -on the work, step by step, as 
directed. In the following instructions it is assumed that the 
eccentric is secured to the shaft by means of set screws, in which 
case the proper position of the eccentric must be determined. 
Thus the instructions will cover nearly all cases of failure of the 
gear and enable the engineer to substitute new parts and to 
set the entire gear correctly without the aid of marks usually 
put on the several parts by the builders. 
Lead and Tools. 
The amount of lead mentioned in this pamphlet is 1-32 of an 
inch, which is sufficient for all sizes of cylinders ordinarily met 
with in practice, excepting low pressure cylinders of compound 
engines, which usually require 1-16 of an inch lead. The tools 
required for marking and setting the gear are: Steel scriber, 
straight-edge, flexible scale 4 or 6 inches long, or a pair of di- 
viders, 2-foot rule, sharp cold chisel and a hammer. 
Putting the Gear in Order. 
Before attempting to set the eccentric or to adjust the position 
of the wristplate and valves of the Corliss engine, the valve 
gear should be put into good working order. By this is meant 
that all unnecessary lost motion should be taken up, and if any 
brasses need renewing or need shimming up, as it is called, this 
should be done ; and all the joints adjusted and properly lubri- 
cated before attempting to adjust the valves, otherwise a sub- 
sequent adjustment of the valve gear might render the adjust- 

Copy right, lyOS, by H. H. Kelley. 



merit of the valves very inaccurate, thus necessitating going 
over a considerable portion of the work a second time. 

The gear should first be cleaned, and the dashpots caused to 
seat properly, provided they do not already do so. The die 
plates on the hooks and catchblocks should be either renewed or 
turned around so as to oirer true, sharp contact edges. The 
lap or the extent of engagement between the hooks and catch- 
blocks should be kept down to as near 1-16 of an inch as possi- 
ble, and both hooks should have the same lap or engagement 
with the catchblocks. The safety device should be placed so 
that the hooks will normally engage the catchblocks v/hen the 
engine is not in motion. Observing these minor details previous 
to attempting to set the Corliss valve gear will save much time 
and annoyance, and in the end will insure a much more accurate 
job, and consequently a better working engine. 

Equalizing the Motion. 

The first work in setting the Corliss valve gear is to equalize 
the movements of the several parts. In order that the same 
quantity of steam may be admitted to and exhausted from each 
end of the cylinder at the proper time, the valves must be given 
equal travel, that is, both steam valves should open at the same 
time with reference to the position of the piston, and the valves 
should travel equal distances in opening the ports. The exhaust 
valves, also, should have equal travel. In order that the valves 
may have equal travel the valve gear must be so adjusted that 
it will have equal travel with reference to a fixed point. This 
point is the plumb line — that is to say, the driving pin in the 
wristplate should move the same distance on both sides of the 
plumb line when the line is suspended in front of the center 
of the wristplate stud or bearing. 

As the reach rod, which connects the wristplate to the rocker 
arm, is seldom adjustable in length, it follows that if the rocker 
arm be placed in a vertical position the wristplate will occupy 
its central position, or its position of mid-travel. As the rocker 
arm is easier to plumb than the wristplate, on account of the 
pins being nearer the line vertically, the rocker arm is first 
placed in its central position, as shown in Fig. 1. The full side 
of the eccentric will now occupy one of the quarters. Pay no 



attention to the position of the eccentric further than to see that 
the center of the full side of the eccentric is about vertical. No 
attention need be paid to the crank or piston, these will be at- 
tended to as the work progresses and each brought into the 
proper position. 

Marking the Wristplate. 
While the rocker arm is in the central position, take a straight- 
edge and scriber and draw a line on the hub of the wristplate, 
extending it to the edge of the wristplate support, which usually 
is flush with the hub of the wristplate. This line will, of course, 
be divided into two parts, one part being on the wristplate hub, 
which part of the line we shall call B, Fig. 1 A, and the part 
on the support is designated A. In order that the wristplate 
may impart equal motion to the two steam valves and equal 
motion to the two exhaust valves, it is evident that the line, B, 
will have to move the same distance both ways from line, A. 
The movement of the wristplate is limited by the eccentric, so 
that when the eccentric reaches the extreme position, the wrist- 
plate will have been moved to the end of its travel. Therefore, 
to measure the travel of the wristplate, move the eccentric in 
either direction as far as it will go, as shown in Fig. 1 B, or 
until it ceases to move the wristplate. With the scriber and 
straight-edge draw a very light line C, on the wristplate hub 
and in line with the one marked A. Measure the distance with 
a flexible scale or a pair of dividers, between lines B and C. 
Then have the eccentric turned to the opposite extreme position 
as shown in Fig. 1 G, and draw another line D, on the wrist- 
plate hub. With the dividers try the distance between lines B 
and D. If the travel of the wristplate is equal on both sides of 
the line A, it is evident that the distances between the lines B 
and C, and between the lines B and D, must be the same. If 
one of these distances is found to be greater than the other, the 
eccentric rod should be adjusted so as to make the shorter dis- 
tance longer; the exact amount to move the eccentric rod will 
be whatever is necessary to move the wristplate an amount 
equal to one-half the difference between the distances BC and 
BD. Then, after trying these distances by moving the eccen- 
tric to the extreme positions, scribe the lines deeper so that the 





n 




1 


""' ■■' - 













corrected positions may be obtained at other times. Both, the 
rocker arm and the wristplate will then have equal travel on 
both sides of the plumb line and consequently will be able to 
impart equal motions to the valves. 

Equalizing the Position of the Valves. 

The next step is to equalize the positions of the valves. The 
positions of the steam valves, when hooked up, and of the ex- 
haust valves, can only be equal or the same when the wristplate 
is in its midtravel position. That position is obtained by mov- 
ing the eccentric until the lines A and B, Fig. 1 A, are exactly 
in line. It is well to secure the wristplate in this position, be- 
cause when making adjustments of the other parts of the gear 
it is oftentimes quite easy to move the entire gear, especially 
when the eccentric is loose on the shaft. The gear may be locked 
by tightening the set screw in the eccentric or by putting in 
a thin washer between the wristplate hub and the washer which 
holds it on the stud, and then tightening the nut. One valve 
should now be partly raised — that is, the hook at one end of the 
cylinder will have engaged the steam valve crank-arm and 
will have raised it a certain distance, and with it the dash- 
pot plunger. The valve arm, which is now disengaged from 
the hook, should be raised and hooked on so that both steam 
valves will occupy the same position at the same time, as shown 
in Fig. 1. 

Marking the Valves. 

If, upon removing the front bonnets from the valve chests, 
no marks are seen on the ends of the valves and on the valve 
seats, take the steam valves out, one at a time, and mark them, 
replacing one valve before removing another, so as not to get 
the valves into the wrong chests. Take a try-square and scriber, 
and, placing the blade parallel to the working edge of the valve, 
as shown in Fig. 2, draw a fine line to the end of the valve; 
then draw a radial line across the end of the valve to meet it, 
as shown in the same figure. After checking up to see that the 
lines are correctly located, take a sharp chisel and make the 
radial line deeper near the edge of the valve. 

In case of multiported valves, select the working edge that 
is nearer the bottom of the chest, because it is easier to measure 



its movements when adjusting the valve. Be sure that corres- 
ponding edges are selected on both valves, as shown in the upper 
view in Fig. 1. While each valve is out, take a straight-edge 
and scriber and draw a line on the valve seat in line with the 
working edge of the port that corresponds to the working edge 




WORKING EDGE LAP 



FIG. 



SHOWING LAP OF VALVE, METHOD OF MARKING AND 
HOOK CLEAR AJSCE. 



marked on the end of the valve. Make the line long enough 
so that it can be seen when the valve is replaced in the chest. 
Mark all the valves and valve seats in this same manner. It 
will no doubt be seen that when the valves are in their proper 
places in the chest, the working edge of both the valves and 
ports can be easily and accurately located by means of the 
marks and the exact position of the valves determined. 

Lap of Valves. 

When the valves are all in place the working edges of the 
steam valves should lap over the working edges of the ports, 
as shown in Fig. 2, the amount of the lap being indicated by the 
distance between the mark on the end of the valve and the 
mark on the valve seat, as shown in the upper view in Fig. 1, 
which shows the ends of both of the valves, which are shown 
in section below. This distance representing the lap varies with 
different sized engines. The amount of the lap to be given the 
steam valves will be found in the accompanying table. 

Take the distance representing the lap between the points of 
the dividers and with one point on the line on the valve seat 
and the other point lying toward the middle of the cylinder, 
have the valve moved until the line on the valve reaches the 



TABLE SHOWING LAP AND TRIAL COMPRESSION. 



SIZE OF 
CYLINDER 


Lap of Steam 
Valves, Wrist- 
plate at 
Midtravel 


TRIAL COMPRESSION 


Minimum 
Steam Valves. 
Double Eccen- 
tric. 


Non-Conden- 
sing Single 
Cyl. 


Comp. Non- 
Cond. L. P. 

Cyl. 


Condensing 


8x24 
10x24 


Inch. 
5—32 


1 1-2 ins. 
1 1-2 


3 ins. 
3 


4 ins. 

4 


Inch. 
5—32 


12x30 
12x36 


3—16 


o 

2 1-4 


3 3-4 

4 1-2 


5 

6 


3—16 


14x36 
14x42 
14x48 


1—4 


2 1-4 
2 3-4 
3 


4 1-2 

5 1-4 
6 


6 

7 

8 


3—16 


16x32 
16x36 
16x42 
16x48 


5—16 


o 

2 1-4 
2 3-4 
o 


4 

4 1-2 

5 1-4 
6 


5 
6 

7 
8 


1-^ 


18x36 
18x42 
18x48 
20x42 
20x48 
20x60 


B-* 


2 1-4 
2 3-4 
3 

2 3-4 
3 

3 3-4 


4 1-2 

5 1-4 
6 

5 1-4 

6 

7 1-2 


6 

7 
8 
7 
8 
10 


1—4 


22x42 
22x48 
22x60 


13—32 


2 3-4 
3 

3 3-4 


5 1-4 

6 

7 1-2 


7 

8 

10 


1—4 


24x42 
24x54 

24x60 
26x48 
26x60 
28x48 
28x60 
28x72 


7—16 


2 3-4 

3 1-2 
3 3-4 
3 

3 3-4 

3 3-4 

4 1-2 


5 1-4 

6 3-4 

7 1-2 
6 

7 1-2 

6 

7 1-2 

9 


7 

9 
10 

8 
10 

8 
10 
12 


9—32 


30x48 
30x60 
30x72 


15—32 


3 

3 3-4 

4 1-2 


6 

7 1-2 

9 


8 
10 
12 


5—16 


32x48 
32x60 
32x72 
34x48 
34x60 
34x72 


1—2 


3 

3 3-4 

4 1-2 
3 

3 3-4 

4 1-2 


6 

7 1-2 

9 

6 

7 1-2 

9 


8 
10 
12 

8 
10 
12 


5—16 


36x48 
36x60 
36x72 
38x60 


17—32 


3 

3 3-4 

4 1-2 
3 3-4 


6 

7 1-2 

9 

7 1-2 


8 
10 
12 

10 


5—16 


40x48 
40x60 
40x72 
40x84 
42x48 
42x60 
42x72 


9—16 


3 

3 3-4 

4 1-2 

5 1-4 
3 

3 3-4 

4 1-2 


6 

7 1-2 
9 
10 1-2 
6 

7 1-2 
9 


8 
10 
12 
14 

8 
10 
12 


3—8 


44x48 
44x60 
44x72 
46x60 
46x72 


5—8 


3 

3 3-4 

4 1-2 

3 3-4 

4 1-2 


6 

7 1-2 

9 

7 1-2 

9 


8 
10 
12 
10 
12 


3—8 


48x60 

48x72 


11—16 


8 S-4 

4 1-2 


7 1-2 
9 


10 
12 


7—16 



8 

point of the dividers. The valve is moved by turning the right- 
and-left adjustment in the radial rod connecting the valve with 
the wristplate. As soon as the valve has been moved into the 
proper position, tighten the jam nuts in the right-and-left ad- 
justment in the radial rod. Adjust the valve at the opposite end 
of the cylinder in precisely the same manner. The lines on 
the backs of the valves and on the seats will then appear in the 
position shown in the upper view in Fig. 1. 

The exhaust valves should be adjusted so that the marks on 
the ends of the valves and on the valve seats are nearly in line, 
allowing not more than one-sixteenth of an inch lap. See lower 
view, Fig. 1. The right-and-left adjustment in the radial rods 
of the exhaust valves may be left loosened as further adjustment 
of these rods will be necessary. Now remove the washer from 
the wristplate hub or loosen the set screws in the eccentric, 
depending upon how the gear has been locked in position, thus 
releasing the gear so that the eccentric can be turned on the 
shaft. Turn the eccentric to one of its extreme positions. See 
that the corresponding dashpot plunger is well seated. The 
clearance, 0, Fig. 2, between the die on the hook and the die 
on the catchblock attached to the crankarm on the valve stem, 
should be about 1-32 of an inch. The clearance is adjusted 
by lengthening or shortening the dashpot rod if need be, 
after which the jamnuts should be carefully tightened. Turn 
the eccentric to the opposite extreme position and adjust the 
clearance at the opposite hook in precisely the same manner. 
Setting the Eccentric. 

The next step is to set the eccentric with reference to the 
crank. Place the crank on the exact dead center — preferably 
the one nearest the cylinder. Turn the full part of the eccen- 
tric to the corresponding position and so that the top of the 
rocker arm will have been moved as far as possible toward the 
head end of the cylinder, which will bring the head end hook 
into position to engage the head end catchblock on the steam 
valve crank-arm. Have the eccentric turned in the same di- 
rection in which the crank is to turn until the valve at the head 
end of the cylinder has opened the port 1-32 of an inch, as shown 
which opening will be indicated by the lines on the 



10 

end of the valve and on the valve seat. These lines should 
then be 1-32 of an inch apart, as shown at the left-hand side 
of the upper view in Fig. 3. Have the eccentric tightened to 
the shaft, when the eccentric will have been properly set. It is 
important that the lead be the same at both ends of the stroke, 
therefore, to ascertain whether the valve at the crank end gives 
1-32 of an inch lead, have the crank turned to the crank-end 
dead center and again measure the lead. If it is more or less 
than 1-32 of an inch, move the valve slightly by adjusting the 
right-and-left connection in the radial rods. 

Equalizing the Lead on Large Engines. 

If the engine is a large one or is of the compound type, it 
will not be convenient to turn the crank from one dead center 
to the other to see whether the lead is equal. In that case, 
after setting the eccentric, as shown in Fig. 3, measure the dis- 
tance between line A and line B, Fig. 1 B, which is the measure 
of the full movement of the wristplate out of a central position. 
Raise the reach rod from the driving pin in the wristplate, and 
by means of the starting bar, have the wristplate moved in the 
opposite direction until the line B is exactly the same distance 
on the other side of line A. It will be understood that the 
wristplate will now occupy the same position that it would if 
the crank had been turned to the crank-end dead center, and 
consequently, the lead may be thus adjusted at the crank end 
without turning the crank. If the re-adjustment for lead 
amounts to 1-32 of an inch, the gear should be put into its 
extreme position and the clearance 0, Fig. 2, again adjusted,. 
if need be, by means of the dashpot rod. 

Adjusting the Exhaust Valves. 

The next step is to adjust the exhaust valves so that they 
will give approximately the proper compression. First look in 
the table of Trial Compression and opposite the given size of 
engine will be found the point in the return stroke at which the 
exhaust valve should close the port. If the crosshead over- 
rides the guides at the ends of the stroke, it will be desirable ta 
place the crank on the dead centers and then mark the position 
of the end of the crosshead on the guide or frame, as at E, Fig. 



12 

4. Beginning at this line, measure off the distance given in 
the table and make another line, F. Do this at the opposite end 
of the guides, as shown at G and H, Fig. 4. 

Starting with the crosshead at one end of the stroke — say at 
the end nearest the cylinder, have the crank turned in the same 
direction in which the engine is to run until the end of the 
crosshead reaches the line F near the end of the stroke, as shown 
in Fig. 4. The exhaust valve that corresponds to this end of 
the stroke, or that which is nearest the piston — in this case the 
crank-end valve — should have just closed the port. The posi- 
tion of the valve can be seen by means of the mark on the end 
of the valve and the mark on the valve seat; these should be 
exactly in line with each other, but if they are not in line, the 
valve may be moved one way or the other by means of the right- 
and-left adjustment in the radial rod which connects the valve 
with the wristplate. After bringing these two lines in line with 
each other, have the crank turned in the same direction as before 
until the crosshead reaches the other line H, near the oppo- 
site end of the guides, as shown in Fig. 5. The opposite valve 
— in this case, the head end valve — should now have just closed 
the port, and consequently, the lines on the end of the valve 
and on the valve seat should be exactly in line with each other. 
This valve may be adjusted the same as the other one, if 
need be, after which the jamnuts in both the radial rods con- 
nected to the wristplate should be tightened and the exhaust 
chest bonnets put on. The steam valve bonnets also may now 
be put on, if desired, because the lines on the valves and seats 
will be of no further use in making the remainder of the adjust- 
ments. 

Regulating the Governor for Safety. 

The next step is to see that the governor will prevent the 
hooks from engaging the catchblocks on the steam valve crank- 
arms when the governor balls are in their lowest position and 
when they are in their highest position. Take out the collar, 
cam or pin, or whatever may be employed to prevent the gov- 
ernor from reaching its lowest position, and let the balls go 
down as far as they will go. Turn the crank to one of the dead 
centers, then examine the hook corresponding to that dead cen- 



13 



ter. If it has raised the steam valve crank-arni, the gover- 
nor rod connected to the knock-off cam or trip plate at that 
end of the cylinder should be adjusted until the crank-arm 
is released from the hook. Put in the starting bar and raise 
the reach rod from the driving pin in the wristplate, then turn 
the wristplate to the opposite extreme position, which will be 
indicated by the marks AC or AD on the wristplate hub and the 
support, being in line with each other. Now examine the other 
hook, and if need be, adjust the governor rod at that end so 
that the hook will just fail to engage the catchblock. 

Next raise the governor balls to their highest position and 
examine both hooks to see that they do not engage the catch- 
blocks on the crank-arms. If one of the hooks only should 
engage the catchblock, examine the trip plate to see that the 
projection which trips the hook is not worn or loose. If it is 
all right, then adjust the governor rod at that end so that the 
hook will just clear the catchblock. In event both hooks en- 
gage the catchblocks, raise the upper collar on the governor 
spindle slightly, if practicable, after raising the balls, and again 
examine the hooks, which will now clear the catchblocks. If 
the collar is fixed and cannot be raised, then adjust the governor 
rods so as to prevent the hooks from engaging the catchblocks. 
It should be stated in this connection that when the hooks fail 
to engage the catchblocks when the governor balls are in their 
lowest position, the hooks will also fail to engage the catch- 
blocks when the balls are in their highest position, except 
when the trip plates are loose or have become badly worn 
and need replacing. It is well, therefore, to try the hooks 
with the balls in both the highest and lowest positions and to 
have the hooks clear by considerable in order to allow for some 
adjustment of the trip plate when equalizing the cutoff. When 
the hooks just clear the catchblocks, when the balls are in their 
lowest position a very slight adjustment of the governor rods, 
as a usual thing, will cause them to clear the catchblocks when 
the balls are in their highest position. 

Equalizing the Cutoff. 

The next and last step is to adjust the trip plates or kncck- 
off cams so that they will produce an equal cutoff on both 



14 

strokes. The average point of cutoff in single eccentric Corliss 
engines when properly loaded is at about one-quarter stroke. 
First divide the length of the stroke expressed in inches by 4, 
and beginning at the lines G and E, Fig. 4, on the guide or 
frame, representing the ends of the crosshead travel, measure 
off one-quarter of the stroke and make a fine line on the guide 
as at J and K, Figs. 4 and 5. Be careful to obliterate the lines 
H and F, for the time being, since the latter lines show where 
the exhaust valve should close ; otherwise there will be two lines 
rather close together, which may prove to be confusing. 

After measuring off one-quarter stroke from both ends of the 
crosshead travel (not the ends of the guide) and properly 
marking the guide at these points, as shown at J and K, Figs. 
4 and 5, have the crank turned to one of the dead centers. Now 
have the crank turned in the same direction in which the en- 
gine is to run until the end of the crosshead nearest the end 
of the guide comes to the line representing one-fourth stroke. 
It is evident that if the cutoff is to occur at this point in the 
stroke, the steam valve corresponding to this end of the cylinder 
should be released from the hook at this point; therefore, raise 
the governor balls slowly until this valve is released. Block 
the governor in exactly this position, have the crank turned in 
the same direction as before until the crosshead reaches the 
end of the stroke and until the end of the crosshead nearest the 
end of the guides comes back on the return stroke to the line 
representing one-quarter stroke. If the valve corresponding to 
this end of the cylinder has not been released from the hook, 
adjust the governor rod slowly until it is just released. Should 
the valve be released before the crosshead reaches the line, then 
the governor rod will be adjusted slightly so as to cause the 
valve to be released later. The crosshead should then be moved 
back a trifle, then move it forward slowly until it reaches the 
line, again noting at what point the valve is released. In this 
way the governor rod may be adjusted so that the valve will be 
released by the time the crosshead reaches the line. The block- 
ing should now be removed from the governor and the safety 
device inserted so that the governor balls will not descend to 
their lowest position. 



15 



It is always well to put in the starting bar and work the 
wristplate back and forth a few times to see that the hooks 
properly engage the catchblocks. 

After the engine is started and all the parts have reached 
their normal temperature, it is a good plan to connect the indi- 
cator and take a few diagrams, which will indicate the effects of 
the expansion of the gear upon the action of the valves. It is 
probable a very slight adjustment of the radial rods or the gov- 
ernor rods may be necessary, but if the work has been correctly 
clone, as previously described, the adjustment of any of the 
parts will be very slight. The directions for setting the exhaust 
valves to give the proper compression, and for equalizing the 
points of cutoff apply also to the double eccentric Corliss gear, 
because after the eccentrics are once set these adjustments are 
made by means of the radial rods and governor rods the same 
as with the single eccentric gear. 



^^^^^^^^^^^^ 



16 



ADJUSTING THE DOUBLE ECCENTRIC GEAR. 



THE double eccentric Corliss engine when first introduced 
was employed principally in street railway service, where 
the load fluctuates through a wider range than in almost 
any other service with the exception of rolling mills. The 
use of two eccentrics provides the engine with what is com- 
monly styled the long range cutoff, because the additional ec- 
centric enables the point of cutoff to be extended to approxi- 
mately 3-4 stroke, thus giving a period of admission nearly 
twice as long as can be obtained in the single eccentric machine. 
The advantages of the long range cutoff are not confined to 
convenience and in enabling the engine to carry very heavy 
overloads. It is well known that with a given initial pres- 
sure and piston speed, the size of the cylinder may be reduced 
as the point of cutoff occurs later in the stroke. A 12 x 36 
Corliss engine in which the cutoff occurs at 1-2 stroke when 
making 117 revolutions per minute will develop as much power 
as a 14 x 36 engine making 100 revolutions per minute in which 
the cutoff occurs at 3-8 stroke. 

The maximum power of the single eccentric engine is reached 
when the cutoff occurs at about 3-8 stroke; therefore, this style 
of engine must be so proportioned that the heaviest load can 
be carried without interfering with the speed regulation when 
the cutoff occurs not later than 3-8 stroke. The double eccentric 
engine may be proportioned to carry the normal load at 3-8 
cutoff, if so desired, because the maximum power of the engine 
is not attained until the point of cutoff occurs at approxi- 
mately 3-4 stroke. It is evident that for equal power and with 
the same percentage of overload to be provided for, the single 
eccentric engine will have a somewhat larger cylinder, which 
incidentally adds to the initial cost of the engine. Taking 
two engines of the same size, one a single eccentric and the 
other a double eccentric engine, when the cutoff occurs at, 
say, 1-4 stroke in each, the single eccentric engine will develop 
80 per cent of the maximum power, while the double eccentric 



17 



engine will develop only 61 1-4 per cent of the maximum power 
attainable, assuming the initial pressure, back pressure and 
piston speed to be the same in both cases. 

As a general thing, the advantages of the double eccentric 
engine with reference to the distribution of steam can be more 
clearly understood by considering the limitations of the single 
eccentric gear. When a person clearly understands why the 
point of cutoff with the single eccentric gear cannot be extended 
beyond a certain point, which, as already stated, is approx- 
imately at 3-8 stroke, he at once understands the more practic- 
able method of extending the point of cutoff and is able to 
comprehend the necessity for the additional gear. The limita- 
tions of the single eccentric gear may be explained as follows : 
Latest Cutoff in Single Eccentric Engine. 

The limit of cutoff or the latest point of cutoff obtainable in 
the single eccentric engine is illustrated in the accompanying 
drawings. Fig. 6 represents the position of the valves, crank 

//, 





FIG. 6. VALVES AND ECCENTRIC AT THE BEGINNING OF THE STROKE. 

and eccentric at the commencement of the stroke. The steam 
valve at the head end of the cylinder has opened the port to the 
extent of the lead, while the eccentric has been turned around 
on the shaft to a point corresponding to the lap and lead of the 
steam valves. When the crank reaches the position shown in 
Fig. 7, the eccentric will have reached the dead center farthest 
from the cylinder and it is evident that any further movement of 
the valves in the same direction is impossible, in other words, 
the valves have reached the extreme limit of their travel. Neg- 



18 



lecting the effect of inertia and recognizing the fact that nothing- 
occurs instantaneously in mechanics, the position of the piston 
in Fig. 7 indicates the latest point of cutoff, which, it will be 
noticed, occurs at about 3-8 stroke as long as the governor 
remains operative. 





FIG. 7. ECCENTRIC ON ITS OUTWARD DEAD CENTER. 

A very natural question is, why cannot the valves be given 
less lap and the radial rods shortened, thus permitting the ec- 
centric to be set more nearly at right angles to the crank t 
That, of course, can be done, and the radial rods connected to 
the exhaust valve stems can be adjusted to compensate for the 
new position of the eccentric. Fig. 8 represents the cylinder of 




FIG. 8. SINGLE ECCENTRIC ENGINE— VALVES GIVEN LESS LAP. 

an engine in which these changes have been made. Very little 
difference in the position of the parts is noticeable, excepting 
the position of the eccentric, because it is only possible to 



19 



obtain a slight change in this manner even when specially con- 
structed valves are employed. 

It must not be forgotten that both the steam and exhaust 
valves must have some lap in order to render them steam tight 
when closed, and that about 1-8 inch lap is as little as can be 





i'lu. y. JbAXJiiilNTitie OIN l±iE OUTWARD DEAD CENTER. 

depended on. When the crank, illustrated in Fig. 8, is turned 
to the position indicated in Fig. 9, the eccentric will occupy the 
same position as in Fig. 7, and it will be seen that the same 
conditions obtain, viz., the eccentric has reached the dead center 
farthest from the cylinder and the valves have reached their 
extreme position during the outward stroke of the piston. The 

m 



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W^/JYSSSffStYSSSiYSSS/ZZl 




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*ltjr. J.U. Jt-ioxOiN Ai r-uiiM w i.in,±tE COMPRESSION SHOULD BEGIN. 

position of the piston indicates that the cutoff now occurs at 
about 1-2 stroke, which apparently is an improvement. The 
difficulties with this arrangement are experienced on the return 



20 



stroke and also at the end of the outward stroke. Fig. 10 illus- 
trates the position of the valves and piston when release should 
occur, and it is found to be much too late, which is due to the 
fact that by moving the eccentric back or nearer the cylinder it 
becomes necessary to re-adjust the length of the rods operating 
the exhaust valves so that the valves may occupy the proper 
position at the commencement of the stroke. This adjustment 
has resulted in the lengthening of the rod at the head end of the 
cylinder and shortening the one at the crank end. Fig. 11 illus- 
trates the conditions obtaining when the piston reaches the 
end of the stroke. 





FIG. 11. PISTON AT THE END OF THE OUTWARD STROKE. 

The exhaust valve at the crank end of the cylinder has not 
quite closed the port, indicating that no compression can be 
obtained, and that release at that end of the stroke will take place 
much too early. The exhaust valve at the head end of the 
cylinder has not yet opened the port, showing that release 
is too late, and not only that, but it is evident from the position 
of the eccentric that the valve cannot open the port wide, hence 
the port will be closed earlier during the return stroke, thus 
producing too much compression. 

Equalizing the Motion. 

When about to set the double eccentric Corliss valve gear,, 
the same care should be exercised in getting the gear into proper 
working order — that is, with reference to the lap of the die 
plate on the hooks and catchblocks, seeing that the gear and 
dashpots are clean and free from all unnecessary lost motion. 



and that the dashpot plungers seat properly. Measure the width 
of the steam port which is to be uncovered by the valve, and. 
note it on paper. 

The motion or travel of the valves must be equalized the 
same as with the single eccentric gear so that the two steam 
valves and the two exhaust valves will have equal lengths of 
travel. The method of equalizing the motion of each of the 
wristplates is precisely the same as with the single eccentric 
gear, therefore, it will be unnecessary to repeat the directions. 
First equalize the travel of the steam wristplate, then equalize 
the travel of the exhaust wristplate, marking the extreme posi- 
tions and the central position of each wristplate, by means of 
lines on the wristplate hub and its support, the same as with, 
the single eccentric gear. 

Adjusting the Position of the Valves. 

The first step, after equalizing the motion of the wristplate, 
is to turn the steam eccentric on the shaft so that the marks. 
A and B, on the wristplate hub and stud are exactly in line 
with each other, as shown in Fig. 1 A. Tighten the set screws, 
in the steam eccentric or otherwise lock the wristplate firmly in 
this position. If the edges of the valves and ports are not indi- 
cated by means of lines on the ends of the valves and on the 
valve seats, pull out one valve at a time and put on the marks 
as described in connection with the single eccentric gear. After- 
marking the valves and the valve seats and replacing the valves, 
see that the hook has engaged the crank arm and has raised it, 
as in Fig. 1. The marks, in the case of the double eccentric 
gear, should indicate that the valve has opened the port about 
one-fourth of the width of the port, which opening will be indi- 
cated by the line on the end of the valve and on the valve seat- 
being one-fourth of the width of the port apart. If the port 
is opened too wide or not wide enough, move the valves by ad- 
justing the right-and-left connection in the radial rods. Re- 
lease the wristplate and pick up the opposite valve and adjust 
its position while the wristplate is at mid-travel. 

Raise the steam reach rod and move the steam wristplate to 
the end of its travel, which position will be indicated by the 
lines AC or AD, being in line with each other. Next look at 



the hook, which is now in position to engage the catchblock on 
the crank-arm. Lengthen or shorten the dashpot rod until the 
clearance between the die plates on the hook and on the catch- 
block is about 1-32 of an inch. Next look at the line on the 
end of that valve and note the lap, which will be indicated by 
the line on the valve being now on the opposite side of the line 
on the valve seat, the distance between these lines being a little 
less in this direction than when the wristplate occupied its cen- 
tral position. If the lap is too small, the minimum lap being 
about 1-8 of an inch, adjust the position of the valve by means 
of the radial rod which operates it, afterward adjusting the 
length of the dashpot rod so as to hold the valve in this position. 
Then give the proper clearance between the die plates on the 
hook and catchblock by adjusting the radial rod. Turn the 
wristplate to the extreme position in the opposite direction, and 
adjust the opposite steam valve in precisely the same manner. 
Setting the Steam Eccentric. 

The next step is to place the crank on the dead center, pref- 
erably the one nearest the cylinder. Turn the full part of the 
steam eccentric to its dead center or as far as it will go toward 
the cylinder. Now note the kind of valve gear employed. If 
it is of the style shown in Fig. 12, turn the steam eccentric on 
the shaft in the same direction in which the engine is to run 
until the steam valve at the head end of the cylinder has opened 
the port 1-32 of an inch. Tighten the set screws in the ec- 
centric. 

Measure the distance the wristplate has been moved from 
its extreme position, which distance will be indicated by the 
lines on the wristplate hub and its support. Lay off this distance 
on the wristplate support from the extreme position in the oppo- 
site direction, and mark it, then turn the wristplate so that the 
line B, on the wristplate hub, comes in line with the one just put 
on the support. The wristplate will now occupy the same po- 
sition that it would if the crank had been turned to the op- 
posite dead center. The other steam valve — that is. the valve 
opposite to the one first adjusted — should now have opened 
the port 1-32 of an inch. Its position may be adjusted, if need 
be, by means of the radial rod. 



23< 





/k A 





24 



Adjusting the Exhaust Valves. 

The next step is to have the exhaust eccentric turned round 
on the shaft until the exhaust wristplate reaches its central 
position, which is indicated by means of the marks A and B on 
the wristplate hub and its support. These lines should be ex- 
actly in line. The lines on the ends of the exhaust valves and 
on the valve seats should first be brought exactly in line with 
each other for speeds up to 90 revolutions a minute, and for 
higher speeds they should be given 1-16 of an inch lap as the 
trial position. 

The next step is to set the exhaust eccentric. First place the 
crank on the dead center nearest the cylinder. The more con- 
venient starting point in this case is to have the exhaust wrist- 
plate in its central position, and have the full side of the eccen- 
tric on the quarter — that is, either above or below the shaft, 
depending on the style of the gear employed. If the tops of 
the valves move toward the exhaust pipe when opening the 
ports, which is generally the case, then the full side of the ex- 
haust eccentric will be placed according to the connections of 
the reach rod with the wristplate. If the exhaust reach rod is 
connected to the wristplate below the center pin on which the 
wristplate swings or oscillates, as shown in Fig. 13, then the 
full side of the exhaust eccentric will be turned to the lower 
quarter or below the shaft. When the reach rod is connected 
to the wristplate above the center pin, as shown in Fig. 12, 
then the full side of the eccentric will be turned to the upper 
quarter or above the shaft. Having inspected the connection of 
the reach rod to the wristplate, turn the eccentric to the approx- 
imate quarter, hook on the reach rod, set the wristplate in its 
central position by moving the eccentric and secure it in this 
position in such manner that the crank can be turned without 
moving the exhaust gear. 

Adjusting the Exhaust Valves for Compression. 

Next lay off on the guides the compression marks H and F, 
Fig. 4, in the manner described for the single eccentric gear. 
Then have the crank turned in the same direction in which the 
engine is to run until the end of the crosshead nearly reaches 
the opposite end of the stroke and comes up to the line on 



25 



the guide. Loosen the exhaust valve gear, and turn the ex- 
haust eccentric so that the valve which is now nearest the 
piston will just close the exhaust port, or in other words, so that 
it is in position to open the port, the edges being in line. Tighten 
the eccentric to the shaft, then turn the crank in the same 
direction as before until the crosshead comes back on the return 
stroke and reaches the line on the guide. The opposite exhaust 
valve should now have just closed the port. If this valve needs 
moving it may be adjusted by means of the right-and-left con- 
nection in the radial rod. Compression will then begin when 
the piston reaches the points in the stroke corresponding to 
the position which the crosshead now occupies. That com- 
pletes the setting of the eccentrics of the double eccentric 
Corliss gear, and all that remains is to see that the governor 
prevents the hooks from engaging the catchblocks when the 
governor balls are in their lowest and highest positions, and to 
equalize the points of cutoff, which is accomplished in precisely 
the same manner as with the single eccentric Corliss gear pre- 
viously described. 

Styles of Corliss Gears. 

Referring to the diagrams, Figs. 12 and 15, it will be seen 
that there are two styles of Corliss valve gear employing the 
wristplate ; the principal difference is that in one style the radial 
rods are connected near the top of the wristplate, and in the 
other they are connected at the sides. It will also be noticed 
that, in Fig. 12, the top of the wristplate moves in the same 
direction as the piston when opening the steam port at the head 
end of the cylinder, but in Fig. 15 the top of the wristplate 
moves in the opposite direction to the piston when opening 
the same port. It is evident, therefore, that some change in the 
position of the steam eccentric relative to the crank will be 
necessary in order to produce the two motions. When the gear 
shown in Fig. 15 is employed the eccentric should prefera- 
bly be placed at the opposite dead center to the crank as the 
starting point and then turned in the same direction in which 
the engine is to run until the valve at the head end of the 
cylinder opens the port 1-32 of an inch. 

By inspecting Fig. 17 closely it will be seen that, when the 
style of gear shown in the drawing is employed and when the 



26 







UJ 



27 



reach rod is connected below the wristplate stud or bearing, in- 
stead of above it, the motion of the steam eccentric when open- 
ing the port at the head end of the cylinder will be the same 
as in Figs. 1 and 12, and hence the eccentric will start from the 
same dead center and will be turned in the same direction as in 
Figs. 1 and 12. The tops of the steam valves in most Corliss 
engines turn toward the middle of the cylinder or toward the 
steam pipe when admitting steam, and when this is the case 
the eccentric in the two styles of Corliss gears will be turned 
in the directions, and from the starting points, indicated in the 
diagrams, Figs. 12 to 17, provided the piston in all cases is 
at the head end of the cylinder as a starting point, when turn- 
ing the steam eccentric, and the crosshead at the compression 
line on the guides when turning the exhaust eccentric. It 
will be understood that in cases where the direction of rotation 
is reversed and is " under" instead of ''over," the directions in 
which to turn the eccentrics and the starting points for the 
eccentrics will be reversed from those indicated in the diagrams 
and drawings. 

The diagrams, Figs. 12 to 17, represent the two styles of 
Corliss valve gear employing the wristplate motion, the gears 
being in the position they occupy when properly set and with 
the crank on the dead center nearest the cylinder. In the dia- 
grams, Figs. 12 to 17, the steam eccentric and gear are marked 
8, and the exhaust eccentric gear, E. For convenience the 
crank in all the diagrams is represented on the dead center 
nearest the cylinder, and the direction in which it is to turn is 
shown by the large arrow, this direction being known as ' ' over, ' y 
or running over. The starting point which will be found least 
confusing when moving the eccentrics is indicated by dotted 
lines. In Fig. 14, the steam eccentric is first placed on the 
center toward the cylinder, and the exhaust eccentric in the 
opposite position. Then when setting the steam eccentric it is 
moved toward the right to the position indicated by the full 
center line S, the smaller arrow showing the direction in which 
the eccentric is moved. As previously described, the exhaust 
eccentric is not set while the crank occupies the dead center, 
but instead, when it is at a point corresponding to the point in 



28 





A 





29 



the stroke where compression begins. But the exhaust eccentric 
is moved from the starting point in the direction indicated by 
the small arrow. 

Fig. 15 shows another style of gear. With this gear the 
starting point for the steam eccentric is on the center farthest 
from the cylinder, while the exhaust eccentric is started from the 
lower quarter. In Fig. 16 the starting point for the steam ec- 
centric is on the center farthest from the cylinder, and for the 
exhaust eccentric, the opposite center. In Fig. 17 the starting 
points are just the reverse of those in Fig. 16. The object 
in having these starting points is merely to simplify the work 
for persons who are not thoroughly familiar with the different 
styles of Corliss gear. The starting points can be located at 
a glance and when once the eccentric is in the proper starting 
position, it needs but a slight movement to bring it into the 
correct and final position. The causes for the different positions 
of the eccentrics are found at the wristplates, the connections 
to the valves being such that the motions indicated by the small 
arrow must be obtained in order to properly open the valves. 
The diagrams represent about all the different arrangements 
for rocker arms, wristplates and radial rods ordinarily em- 
ployed. With Corliss gears in which no wristplates are em- 
ployed, the steam eccentric is set in the same manner as the 
single eccentric gear shown in Fig. 1, while the exhaust eccen- 
tric is set in the same manner as that illustrated in Figs. 13 and 
14, 15 and 17. 

To Place the Crank on the Exact Dead Center. 

'To place the crank on the exact dead center, put a punch 
mark or a line A, Fig. 18, on the guide near the crosshead, 
and from 1-4 to 1-2 inch from the end of the crosshead travel. 
Procure a strip of wood about 3 feet long, then sharpen a nail 
to a fine point and drive it through the strip at one end. Have 
the crank turned in the same direction in which the engine is to 
run until the crosshead reaches the line near the end of the 
■guide. Now place the strip in a vertical position close to the 
crank disc or the rim of the flywheel, and with the point of the 
nail make a line C near the edge. Have the crank turned past 



30 



the center, as indicated by the dotted lines, and until the same 
end of the crosshead comes back to the line A near the end of 
the guide. Place the strip of wood in the same position as 
before and with the point of the nail make another line B, near 
the edge of the disc or flywheel. 




FIG. 18. PLACING THE CRANK ON THE DEAD CENTER. 

Take a pair of dividers and find the center between the two 
marks on the disc or wheel. At this point make a third mark 
D. Then have the crank turned slowly until the middle mark 
D reaches the point of the nail, the strip of wood being placed 
in the same position as before. The crank will then be on the 
exact dead center. The dead center at the opposite end of the 
stroke is found in the same manner. 

Adjusting the Cutoff for the Average Load. 

Probably it has been noticed by persons in charge of Corliss 
engines that when the cutoff is equalized when taking steam, 
say, one-eighth stroke, it will not be equal when the cutoff 
occurs at three-eighths stroke. It is practically impossible to 
obtain an exactly equal cutoff at all points in the stroke, es- 
pecially in engines fitted with the long range cutoff, in which 
the period of admission frequently varies from zero to three- 
fourths stroke. This is due to the fact that the angle between 
the center line of the actuating arm on the governor columns and 
the center line of the governor rod is not the same when cutoff 
takes place at one-eighth and three-eighths stroke. Referring to 
Fig. 19, it will be seen that the actuating arm is shown in its 
middle position and that the angle between it and the governor 
rod is approximately a right angle. Under these circumstances 
the motion imparted to the governor rod will be equal to the- 



31 



motion of the arm, but if the angle be changed from a right 
angle to an acute or obtuse angle, the motion imparted to the 
governor rod will not be exactly the same as that of the 
actuating arm. Therefore, if the cutoff be exactly equalized 
when the actuating arm occupies its midtravel position, as in 
Pig. 19, when the load becomes such as to cause it to occupy 
either of the positions indicated by the black dots at the ends 
of the arcs representing its extreme travel, the cutoff will not be 




FIG. 19. GOVERNOR ARMS AT MID-TRAVEL. 

•exactly equal. For this reason if the engine carries a fairly steady 
load for the greater part of the time it is well to raise the gov- 
ernor balls to their usual running position when equalizing the 
eutoff. While the differences in points of cutoff due to the gov- 
ernor occupying its extreme positions are slight and for that 
reason have an unimportant influence on the economy of the 
engine, nevertheless this discrepancy in the steam distribution 
is frequently aggravated by a badly working dashpot, which 
allows the valve too much time in closing, thus increasing the 
period of admission considerably beyond that due merely to the 
action of the releasing gear. This is apt to cause a badly dis- 
torted diagram and to lead an engineer to believe that the gear 
is considerably out of proper adjustment. The average or usual 
height of the governor can be obtained by marking the column 
below the central weight when the engine is working under the 
average load, then raising the weight up to this line before 
equalizing the cutoff. The engine will then cut off equally un- 
der average conditions of load and the maximum economy ob- 
tained under all loads. 



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